This application claims priority from International Application No. PCT/EP 99/01817 filed on Mar. 18, 1999, which was published in English as WO 99/49097 on Sep. 30, 1999, and which in turn claims priority from FP 98400721.1 filed on Mar. 26, 1998.
The present invention relates to an organic substrate having optical layers deposited by magnetron sputtering, and to a method for preparing it.
In the ophthalmic field, substrates in transparent organic materials obtained, for example, by polymerizing acrylic, allylic, methacrylic or vinylic monomers are in everyday use. Deposition of thin films on lenses, such as ophthalmic lenses, in such organic materials, or in inorganic materials is also in common use. This for example makes it possible to provide anti-reflective treatment, by successive deposition of different thin films. It is necessary, in such applications, to accurately control the nature and thickness of the various layers deposited.
Conventionally, deposition of thin anti-reflective layers on ophthalmic lenses is done by vacuum evaporation. An organic substrate to be coated with or without an anti-abrasive layer, is placed in a vacuum chamber and the material to be deposited is thermally evaporated by heating or by electron bombardment. In order to improve adhesion of the thin films obtained, the substrates to be coated are heated. In the case of an organic material, the substrate must not be heated beyond 100xc2x0 C.
Such a technique suffers from disadvantages. It is difficult to automate and does not allow continuous flow coating of substrates. Furthermore, stability and reproducibility of the method are not strictly guaranteed.
The layers of the anti-reflective stack can also be deposited by radiofrequency oxide sputtering, but the low rate of deposition renders this technique barely suitable for industrial use. Moreover, this type of method is poorly adapted to the use of targets of large dimensions, thereby limiting the size of the coated substrates.
As against this, cathodic sputtering, which is readily automated, makes it possible to coat substrates of varying dimensions, using continuous or semi-continues flow, while simultaneously ensuring the process is stable. It has thus been proposed to deposit thin films using magnetron sputtering at direct current (DC). This technique consists in vaporizing a solid target which has been brought to a negative potential by the action of a plasma, typically of an inert gas such as argon. Particles detached from the target are deposited on the surface to be coated at a rate, and density, which are far superior to those obtainable with low temperature vacuum deposition. The presence of a magnetic field close to the target, having lines of force parallel to the surface of the target, improves deposition rate by increasing the number of atoms that are vaporized; magnets are used for creating such a field close to the target. The operation is performed in an enclosure under high vacuum, and is called magnetron sputtering.
The technique of magnetron sputtering is well suited to the deposition of metals. In optical applications, it is necessary to deposit layers such as ZrO2, SiO2, TiO2, Nb2O5, Al2O3, Ta2O5, HfO2, Pr2O3, Sb2O5, Y2O3, WO3, In2O3, SnO2, Cr2O3 and mixtures thereof. However, these materials, which are non conducting, are poorly adapted to direct current magnetron sputtering. It has thus been suggested to use a metal target and a plasma consisting not only of argon, but also of oxygen, so that the metal atoms detached from the target become oxidized. This reactive sputtering technique is difficult to implement, notably in view of the difficulty of accurately maintaining constant the amount of oxygen in the plasma. Correct operating equilibrium is unstable and contamination of the target causes deposition rate to diminish.
In order to improve stability of the system, it is also been proposed to apply an alternating voltage to the cathode, rather than a DC voltage, of a sinewave or square wave type. On the positive half-cycle, the cathode is discharged. This technique is known as pulsed magnetron sputtering (PMS) and is disclosed in patent DE-A-37 00 633.
In order to still further improve operation of such a system, patent DD-A-252 205 or DE-A-38 02 852 discloses the use of two cathodes. An AC voltage is applied to each cathode, one cathode discharging while the other is being charged up under the effect of the voltage applied for vaporizing the target. This technique, called Double Magnetron Sputtering (DMS) or Twin Mag, makes it possible to avoid electric arcs.
In order to avoid excessive contamination of the target by oxygen, during sputtering, and to ensure sufficient oxidation of the layer at substrate level, it has also been proposed in patent DD-A-239 810 to regulate the oxygen throughput in the enclosure, as a function of the intensity of a spectral line of the plasma. The intensity of the spectral line emitted by the excited atoms, removed from the target, is proportional to the state of oxidation of the target. This technique is known as Plasma Emitting Monitoring (PEM).
Another method for controlling oxygen flow consists in adjusting the voltage applied to the cathode with respect to a set value. This technique is described in patent DE-A-4106513 or EP-A-501016.
It has been proposed, for example in U.S. Pat. No. 4,572,842, to introduce the reactive gas only close to the surface to be coated, thereby avoiding contamination of the target to be vaporized. U.S. Pat. No. 4,572,842 discloses the use of a septum separating the process cavity into two zones; this patent discloses an example of anti-reflective deposition on a glass substrate; the high inert gas pressure, used in conjunction with a septum, makes it possible to obtain the best reduction of contamination of the metal target by oxygen. Thus, this patent makes it possible to increase deposition rate while still ensuring operation. Patent DD-A-21 48 65 discloses a method for on-line treatment using high inert gas pressure to avoid contaminating the metal target.
It has also been proposed to proceed sequentially with the deposition of a thin metal layer using sputtering followed by oxidation thereof, this technique of sputtering followed by oxidation makes it possible to deposit a metal film under an inert gas such as argon, while preserving rate of deposition. The practical difficulty in implementing this technique is that of preventing contamination of the metal target by the oxygen used for oxidising the metal layer. An apparatus for implementing such a sequential method is disclosed in U.S. Pat. No. 4,420,385: separation of the deposition and oxidation zones which are adjacent, is achieved by means of baffles. Another apparatus is disclosed in EP-A-0,428,358. This apparatus is marketed by Optical Coating Laboratory Inc. (OCLI) under the trade name Metamode(trademark). The deposition and oxygen zones are elongate zones parallel to the axis of the drum carrying the surfaces to be coated. A further apparatus is disclosed in WO-A-92 13114 in the name of Applied Vision Ltd. This apparatus is marketed by the company under the trade name Plasmacoat ARx10(trademark). Oxidation is achieved using a plasma source.
The invention concerns the new problem of adhesion of thin films deposited on a transparent organic material substrate, optionally having received an anti-abrasive treatment, with use of a magnetron sputtering technique. It applies to techniques in which oxidation is sequential as well as to those using sputtering in a reactive atmosphere.
EP-A-0,428,358 mentions, in example 2, xe2x80x9cGlass Eyeglass Lensesxe2x80x9d that the sequential deposition and oxidation method makes it possible to obtain anti-reflective films having good abrasion resistance, for surfaces to be treated in inorganic glass. It does not mention results of tests for example 4, corresponding to anti-reflective deposition on an ophthalmic lens in organic material. WO-A-92 13114 does not mention the problem of adhesion.
The invention proposes, in a surprising fashion, to increase the gas pressure during sputtering of the metallic or semi-conducting target in order to improve adhesion of thin films deposited on a transparent organic material substrate, having optionally an anti-abrasive layer. A prejudice exists against increasing inert gas pressure for sputtering.
In one of the reference works of this subject, Deposition Technologies for Films and Coatings, by Rointain F. Bunshah, it is explained, on page 80, that if a film is deposited by sputtering deposition, atoms removed from the target have energies much higher than thermal energies and may be implanted into the surface to an appreciable depth. The physically incorporated atoms may then act as nucleating and bonding sites for further atom deposition. This passage encourages the person skilled in the art to use high energy atoms for favouring implantation and thus adhesion.
On page 229 of the same work, it is stated that the relatively high energies of the sputtered atoms in magnetron sputtering sources operating at low pressure and in ion beam systems may act to some degree act to promote adhesion by mechanisms similar to those in plasma bombardment. Here again, this general teaching in the relevant art, which would urge skilled worker to chose atoms of high energy.
These teachings incite the skilled worker to chose, for magnetron sputtering, low pressures; indeed, it is known that the energy of vaporized atoms decreases when inert gas pressure increases. In the context, see for example, the article by W. D. Westwood, Calculation of Deposition Rates in Diode Sputtering Systems, Journal of Vacuum Science Technology, 15(1), 1978.
In the state of the art, it is suggested to carry out sputtering at gas pressures of the order of 25 to 80 mTorr, equivalent to 3.3 to 10.64 Pa. In this context, see the work by Bunshah, page 230, for the case of plane diodes and DC current. For such diodes, as there is no plasma confinement by means of the magnetic field, a high inert gas pressure is employed, of the order of 2 to 10 Pa, in order to preserve an intense plasma. This teaching does not apply to magnetron sputtering, for which current pressures are of the order of 0.1 to 0.5 Pa.
In EP-A-0,428,358 in the name of OCLI, it is suggested to operate under an argon pressure of 2.0 micron, equivalent to 2.66.10xe2x88x921 Pa, for deposition of SiO2 and TiO2 on a glass substrate (example 1, table 1). For a substrate in inorganic glass, and films of SiO2 and Ta2O5, it is suggested to operate under an argon pressure of 2.5 micron equivalent to 3.33.10xe2x88x921 Pa (example 2, table 2). For a substrate in organic glass and the same films the proposed argon pressure is also 2.5 micron equivalent to 3.33.10xe2x88x921 Pa (example 2, table 3). The Applied Vision Ltd. apparatus marketed under the trade name Plasmacoat AR10(trademark) suggest working under an argon pressure of 5.10xe2x88x923 mbar, equivalent to 5.10xe2x88x921 Pa for depositing Si, with an argon throughput of 12 sccm. For Zr deposition, it is proposed to operate at a pressure of 8.10xe2x88x923 mbar, equivalent to 8.10xe2x88x921 Pa, with an Ar throughput of 17 sccm.
U.S. Pat. No. 5,170,291 to Leybold discloses a sputtering performed with a magnetron in a reactive gas atmosphere composed of a mixture of Ar and O2. The target materials were Ti, Al, and Si. The pressure during the sputtering was as follows:
sputtering from the Ti-target: 5 10xe2x88x923 mbar
sputtering from the Al-target: 8 10xe2x88x923 mbar
sputtering from the Si-target: 1,2 10xe2x88x922 mbar
Thus, the optical layer that is deposited at high pressure is in this case the low refractive index layer.
The invention proposes, contrary to this practice and teaching, to operate at much higher pressures. It turned out, completely surprisingly, that one can thus improve adhesion of thin films on an organic substrate having optionally an anti-abrasive layer. Also, according to one embodiment, the distance between the substrate and the magnetron is at least 90 mm, preferably between 90 mm and 200 mm, especially between 90 mm and 150 mm. This greater distance between magnetron and substrate allows a lower substrate temperature, which can be highly advantageous, for example when coating temperature-sensitive substrates. Moreover, the layer uniformity, especially on curved substrates, can be further enhanced. If the method of the invention is applied for an inorganic substrate, the excellent results obtained by thermal evaporation are maintained.
More precisely, the invention provides an organic substrate having an optically-active stack on at least one side thereof, characterized in that said optically-active stack comprises at least one optical layer having a high refractive index and at least one optical layer having a low refractive index, at least one of said layers being deposited by magnetron sputtering at high pressure, for example between 0.8 to 5.0 Pa, especially above 1.0 Pa.
Notably, the invention is directed towards an organic substrate having an optically-active stack on at least one side thereof, characterized in that said optically-active stack comprises at least one optical layer having a high refractive index and at least one optical layer having a low refractive index, said at least one high refractive index layer being deposited by magnetron sputtering at high pressure.
Very surprisingly, the adhesion is greatly improved when (only) the high refractive index layer(s) is(are) deposited at high pressure.
Notably, the invention is directed towards an organic substrate having an optically-active stack on at least one side thereof, characterized in that said optically-active stack comprises at least one optical layer having a high refractive index and at least one optical layer having a low refractive index, said at least one high refractive index layer and said at least one low refractive index layer being deposited by magnetron sputtering at high pressure.
Notably, the invention is directed towards an organic substrate having an optically-active stack on at least one side thereof, characterized in that said optically-active stack comprises at least one optical layer having a high refractive index and at least one optical layer having a low refractive index, at least one of said layers being deposited by magnetron sputtering at high pressure between 0.8 to 5.0 Pa, especially above 1.0 Pa, preferably between 1.5 to 3.5 Pa, said sputtering being carried out with the substrate being located at least at 90 mm from the magnetron, preferably between 90 mm and 200 mm, especially between 90 mm and 150 mm,
Specific embodiments correspond to claims 3, 4 and 6 to 18. Specific high sputtering pressure is 1.5 to 3.5 Pa.
The invention applies to substrates having an anti-reflective effect as well as a mirror effect. One or the other effect is obtained by varying the thicknesses of the layers.
The high refractive index is conventionally comprised between 2.0 and 2.6, for example between 2.1 and 2.3, whereas the low refractive index is conventionally comprised between 1.35 and 1.52, for example between 1.35 and 1.46; optionally, the ratio between these two refractive indexes is comprised between 1.5 and 2, for example between 1.5 and 1.7.
The invention also provides a method for producing an organic substrate having an optically-active stack on at least one side thereof, characterized in that said optically-active stack comprises at least one optical layer having a high refractive index and at least one optical layer having a low refractive index, said method comprising the step of depositing at least one of said optical layers by magnetron sputtering at a pressure from 0.8 Pa to 5.0 Pa, for example above 1.0 Pa, preferably 1.5 to 3.5 Pa. The distance between magnetron and the substrate is at least 90 mm, preferably comprised between 90 and 200 mm, especially between 90 mm and 150 mm, this distance being defined as the distance measured between the target and the substrate surface to be coated. Specific embodiments correspond to claims 20 to 35.
In the reactive sputtering in an argon-oxygen mixture embodiment at a pulse rate of from 10 to 100 kHz, the PMS technique as described above may be used. In the pulsed voltage alternating between two neighboring magnetrons embodiment, the DMS technique as described above may be used.
When adjusting the oxygen content of said optical layers by measuring the optical characteristics of the plasma emission, and controlling the oxygen supply as a function of a corresponding signal, the PEM technique as described above may be used. When adjusting the oxygen content of said optical layers by measuring the voltage of the reactive magnetron discharge, and controlling the oxygen supply as a function of a corresponding signal, the method employing comparison with a set value may be used.
The method according to the invention is suitable for the preparation of a substrate according to the invention.
The invention also relates to the use of this method for improving adhesion on thin films deposited on an organic material substrate.
The invention relates finally to the substrate obtained by the method according to the invention especially to an ophtalmic lense.